Influences of High-Basicity High-Alumina Refining Slags on the Cleanliness and Control of Non-metallic Inclusions in Inconel 625 Superalloy

Inconel 625 is a corrosion and oxidation-resistant nickel-based alloy with potential applications in boilers and pressure vessel. However, its corrosion resistance and mechanical properties are related to its purity. Unfortunately, fabrication of Inconel 625 alloy with high cleanliness and competitive cost remains a research and engineering challenge. With that, we report a new study on high-basicity slag-smelting technique used to improve the purity of Inconel 625. More specifically, the effect of slag basicity and reaction time on the cleanliness and the control of oxide inclusions in Inconel 625 alloy were thoroughly investigated. The results showed that slag B with higher basicity possesses stronger ability in improving cleanliness and removing inclusions. Moreover, the inclusions in Inconel 625 alloy refining using slag A mainly and consistently composed of MgO-Al₂O₃. In the case of refining using slag B, however, three different types of impurity inclusions were formed, including MgO, binary MgO-Al₂O₃, and ternary CaO-MgO-Al₂O₃ in the refined Inconel 625 alloy. A more detailed analysis based on thermodynamic calculations revealed that MgO-Al₂O₃ can be spontaneously formed in the alloys during the smelting using both slag systems. However, the content of MgO in slag B is closer to saturation, which contributed significantly to the formation of MgO-based inclusions.